The present invention relates to laser driver circuits, and more particularly to a laser diode impulse circuit that provides a failsafe feature against single point failures and suppresses noise in a current source for the laser diode.
Short duration optical pulses are desirable in many applications, such as photodiode risetime testing, high resolution time domain reflectometry and dispersion and bandwidth testing on optical fiber. Pulse durations of less than forty picoseconds are sometimes desired. To achieve such short duration pulses a laser drive circuit is desired that can produce a very short impulse current pulse. One such circuit is described in U.S. Pat. No. 4,736,380 issued on Apr. 5, 1988 to Agoston Agoston entitled "Laser Diode Driver." In that circuit a reverse current transmitted through an initially forward biased step recovery diode causes the step recovery diode to switch from the forward biased state to a reverse biased state that develops an abruptly rising reverse bias voltage across the step recovery diode. The abruptly rising reverse bias voltage is applied across a series combination of a capacitor and a laser diode connected in parallel with the step recovery diode to force a short, abrupt forward current pulse through the laser diode. In response to the current pulse the laser diode emits a short optical pulse.
The current applied to the step recovery diode is generated by a current source to which an impulse trigger is applied. Due to the complicated nature of the drive circuit with numerous resonating elements, when the current reverses a considerable amount of transients and noise are generated, which transients and noise appear in the short current pulse that is applied to the laser diode. This "trash" results in an undesirable optical pulse shape from the laser diode. Another problem occurs if the capacitor shorts, in which case the current is applied continuously to the laser diode so that the laser diode operates in a continuous wave mode, resulting in a safety hazard due to the laser beam power being at a level that could cause eye damage, i.e., Class III Laser.
What is desired is a laser diode impulse circuit that eliminates single point failures that result in the abatement of laser safety hazards to Class I operation, and that produces a clean, predictable output optical pulse.